Just to add my 2p, I always incorporate a battery grip into my purchase of the camera. I've got large hands so need it and it adds balance to the camera with heavier lenses. I've always bought Canon grips, but I have to say that I don't think the price is very fair considering what it is. I used to think that the ones for the 300D, 30D and 60D were priced about right as you could pick them up for about £100-£130 new each time. With the 5dmk3 the price jump is horrendous. I know they price things with a mind to separating the lower end of the market from the higher, but we've just splashed out £3k on a camera from them so it seems a bit harsh!
Still, there's enough stress in my life without having 3rd party problems, so I just stick to Canon brand accessories for my kit and in 8-9 years I haven't had a single issue.